In the welding together of plastic material it is important to adapt the supply of energy in such a manner that a tight and mechanically strong sealing joint is achieved. The sealing is accomplished in that two or more layers of plastic material are combined and are pressed together by means of a sealing element with simultaneous supply of heat or heat-generating energy, when the plastic material is caused to melt and its combined surface layers are fused together.
If the energy supplied is insufficient the tightness and strength are inadequate owing to the energy supplied not being capable of heating the plastic material to such a degree that a continuous fusing together of the layers pressed against each other is achieved. On the other hand, if the energy supply is excessive, burning of the material may occur, or else the material may be melted down to such a degree that it is removed from the location of the seal which consequently will be thinner and less strong.
In the sealing of laminated material comprising nonthermoplastic layers,such as e.g. laminate consisting of plasticcoated cardboard, the problem is not so pronounced as in the case where the material intended for sealing consists of pure plastic material or, in extreme cases, foamed plastic material.
In the sealing of expanded plastic materials, so-called foamed plastic material,which consists of a large amount of small cells with thin walls of plastic material,there is great need of the correct amount of energy being supplied to the sealing zone. The thin walls cannot be subjected to high pressure, since they would then be crushed together or "collapse". Such a collapse occurs too when they are exposed to excessive heat, since the thin cell walls will then melt down.
It has been known previously that sealing may be carried out by means of sealing elements which give off a certain defined amount of energy. This is done, for example, in the so-called "spin welding process", where the heat generated is in the form of frictional heat. "Spin welding" is carried out in such a manner that two pieces of plastic which are to be combined (usually two cup-shaped parts which are to be joined to a container) are rotated in relation to each other and that they are brought together while still rotating. The braking of the rotating parts, of which at least one is coupled to a flywheel with a certain inertia, generates frictional heat which is sufficient for joining the plastic parts to each other. By adjusting the speed of rotation etc. it is thus possible to determine accurately the energy which the rotating part has, and the whole of this kinetic energy is braked and is transformed to thermal energy when the plastic parts are brought together.
It is also known that with the help of electric contact breakers or regulators the length of the heat pulses can be adapted in such a manner that a certain defined energy is given off from a sealing element. In most cases these regulators are sufficient for adjusting the energy supplied, in particular when the contact pressure in each case of sealing is the same.
In certain cases, however, it is difficult to achieve conditions which given constant sealing pressure, e.g. in cases where the sealing objects are placed in a chain or a line of forming spaces connected to one other. It is difficult, for example, to make the forming spaces exactly like one another or to form the sealing objects in exactly the same manner. This may mean that the parts intended for sealing, when they are brought into sealing position and into contact with sealing elements, may receive different sealing pressure. It has been found that the sealing pressure has a strong influence on the energy supply, especially if the energy is supplied in the form of ultrasonics where high-frequency mechanical vibration produces heating of the material in a manner which partly resembles the earlier "spin welding process", that is to say the mechanical energy supplied is converted to frictional heat in the contact zone between the materials. Since the mechanical energy is not transmitted equally effectively when the contact pressure is low, a longer sealing time is required in such a case for the same energy to be generated in the sealing zone. In order to obtain the same sealing result and the same amount of energy supply it is necessary, therefore, to adjust the sealing time to the contact pressure, but since the packing containers intended for sealing, which arrive in a line after one another, are not exactly the same or placed in exactly the same way into their holders, the contact pressure may vary between packages following each other, so that there has to be an individual regulation in each sealing instance.